The invention relates to a display unit, and more particularly, to a display unit employed in an organic light emitting display device.
FIG. 1 is a schematic diagram of a conventional organic light emitting display panel. As shown, the panel 1 comprises a data driver 10, a scan driver 12, and a display array 14. The data driver 10 controls a plurality of data lines D11 to D1n and the scan driver 12 controls a plurality of scan lines S11 to S1m. The display array 14 is formed by multiple data lines D11 to D1n and multiple scan lines S11 to S1m. The intersecting data scan lines correspond to one display unit. For example, data line D11 and scan line S11 intersect to form a display unit 100. As with any other display unit, the equivalent circuit of the display unit 100 comprises a switch transistor T10, a storage capacitor Cs1, a driving transistor T11, and an organic light-emitting diode (OLED) D1. The driving transistor T11 is a PMOS transistor, for example.
The scan driver 12 sequentially outputs scan signals to scan lines S11 to S1m to turn on the switch transistors within all display units corresponding to one row and turn off the switch transistors within all display units corresponding to all other rows. The data driver 10 outputs video signals with gray level values to the display units corresponding to one row through the data lines D11 to D1n according to prepared image data not yet displayed. For example, when the scan driver 12 outputs a scan signal to the scan line S11, the switch transistor T10 is turned on, the data driver 10 outputs a corresponding video signal to the display unit 100 through the data line D11, and the storage capacitor Cs1 stores the voltage of the video signal. According to the stored voltage in the storage capacitor Cs1, the driving transistor T11 provides a driving current Id1 to drive the OLED D1 to emit light.
Since the OLED D1 is a current-driving element, brightness of the OLED D1 is determined by the intensity of the driving current Id1. The total brightness of the OLED D1 in a frame cycle is the light-emitting intensity thereof. The driving current Id1 is a drain current of the driving transistor and refers to the driving capability thereof. The driving current Id1 is represented in the following formula:id1=k(vsg1+vth1)2 where id1, k, vsg1 and vth1 represent a value of the driving current Id1, a conduction parameter of the driving transistor T11, a value of the source-gate voltage Vsg of the driving transistor T11, and a threshold voltage of the driving transistor T11 respectively.
As shown in FIG. 1, in conventional organic light emitting display devices, the voltages Vdd1 and Vss1 of each display unit are tied together respectively and provided by an external power supply system. The magnitude of the voltage Vdd1 determines the source-gate voltage Vsg of the driving transistor of display units, and the voltage Vss1 is provided to maintain driving transistor T11 operation in the saturation region. Voltage Vss1 must meet the following condition: Vds<Vgs−Vth1. However, the drain voltage Vd of the driving transistor T11 must be evaluated in the presence of the organic light emitting diode D1 between the voltage source Vss1 and driving transistor T11. Using display unit 100 as an example, the drain voltage Vd of the driving transistor T11 meets the condition Vd=Voled+Vss1 wherein voltage Voled represents the turn-on voltage of the OLED D1. However, since the turn-on voltage Voled of OLEDs increases with time, the magnitude of voltage Vss1 is provided with consideration of turn-on voltage increment of OLEDs. Thus, the voltage Vss1 equals [Vdd−Voled−(the turn-on voltage increment of D1)], enabling the drain voltage Vd of the driving transistor T11 to meet the following condition: Vds<Vgs−Vth1. Nevertheless, in practice, the turn-on voltage of OLEDs does not increase at the beginning but after time, and the turn-on voltage increment of better organic light emitting diodes is not at a great amount. Thus, providing a fixed voltage Vss1 with high magnitude results in extra power consumption, more serious when employed in small products.